Modbus can not really disconnect from the port

Sorry for my bad English. I'm from a different country

I connect to the port through the modbus protocol. After working in the program, I need to disconnect from the port and I turn off. There is no mistake. But if you connect through another program to the same port, it turns out that the port is already busy. My program has not really disconnected from the port. If you close my program - the port is freed.

Disconnect method: client->disconnectDevice();
сlient is of type QModbusClient